1. Field of the Invention
The present invention relates to a displacement measuring apparatus for measuring a displacement of an object by irradiating a laser beam to the object, and more particularly, to a displacement measuring apparatus adapted to optically modulate, using a carrier signal of a predetermined frequency, either of laser beams for irradiation to a reference mirror and an object whose displacement is to be measured, respectively, and measure a displacement of the object on the basis of an interference signal produced by interference between reflected laser beams from the reference mirror and object, respectively.
2. Description of Related Art
Heretofore, displacement measuring apparatuses have been proposed which make the use of the laser interference technology to detect a vibration and moved position of an object in a non-contact manner. As one of such displacement measuring apparatuses, a fringe counting type one is generally used.
Referring now to FIG. 1, there is illustrated in the form of a schematic block diagram a conventional displacement measuring apparatus of the fringe counting type.
In FIG. 1, the conventional displacement measuring apparatus is generally indicated with a reference 100. As shown, the apparatus 100 comprises a probe 102 to produce an RF signal by irradiating a laser beam to an object 101 whose displacement is to be measured and modulating a laser beam component developed due to the displacement of the object, a mixer circuit 103 to convert the RF signal to an IF signal, a bandpass filter (will be referred to simply as "BPF" hereinafter) 104 to filter the IF signal, a fringe detection circuit 105 to detect a fringe component of the IF signal filtered by the BPF 104, an up/down counter (will be referred to simply as "U/D counter" hereinafter) 106 to count the detected fringe component and produce a digital displacement signal, a digital/analog converter (will be referred to simply as "D/A converter" hereinafter) 107 to convert the digital displacement signal from the U/D counter 106 to an analog displacement signal, an carrier signal oscillator (will be referred to simply as "carrier OSC" hereinafter) 108 to produce a carrier signal, and a local carrier signal oscillator (will be referred to simply as "local OSC" hereinafter) 109 to produce a local carrier signal on the basis of the carrier signal from the carrier signal oscillator 108.
The probe 102 comprises an interference optical system 110 incorporating a laser diode 111 to emit a laser light, beam splitter 112 to split the laser light into two beams and combine reflected split laser beams, reference mirror 113 and a light modulator 114 to optically modulate the laser beam, which interference optical system 110 producing an interference light by interference between a reflected laser beam from the reference mirror 113 and a one from the object 101, and further a photodetector 115 to detect the interference light and photoelectrically convert it.
The laser diode 111 emits a laser light of a predetermined wavelength. The emitted laser light is split by the beam splitter 112 into two beams. One of the split laser beams is irradiated to the reference mirror 113 fixed inside the probe 102, while the other beam is irradiated to the object 101 through the light modulator 114 which will optically modulate the laser beam irradiated to the object 101 using a carrier signal supplied from the carrier OSC 108.
The beam splitter 112 works also to combine a reflected laser beam from the object 101 and a one from the reference mirror 113 to produce an interference light. The photodetector 115 detects the interference light produced by the beam splitter 112 and optically converts it.
The probe 102 having the above-mentioned construction modulates, using the carrier signal, a laser beam component developed due to a displacement of the object 101 to produce an RF signal, and delivers it at the photodetector 115.
The mixer circuit 103 is supplied with the RF signal from the probe 102, and converts it to an IF signal of an intermediate frequency on the basis of a local carrier signal from the local OSC 109.
The BPF 104 is supplied with the IF signal from the mixer circuit 103 to filter an IF signal component in a predetermined frequency band.
The fringe detector circuit 105 is supplied with the filtered IF signal from the BPF 104, and detects a fringe component of the IF signal on the basis of the carrier signal supplied from the carrier OSC 108. Namely, the fringe detector circuit 105 detects an interference fringe component of the interference light produced by interference between the reflected laser beams from the reference mirror 113 and object 101, respectively.
The U/D counter 106 counts up or down the interference fringe component detected by the fringe detector circuit 105 to detect a displacement of the object 101, and provides the result of detection as a digital displacement signal.
The D/A converter 107 converts the digital displacement signal to an analog signal.
The conventional displacement measuring apparatus 100 of the fringe counting type having the aforementioned construction functions as will be described below:
Since the reference mirror 113 is fixed inside the probe 102, the distance l.sub.1 between the beam splitter 112 and reference mirror 113 will not vary. On the other hand, since the object 101 moves in parallel with the irradiated direction of the laser beam, the distance l.sub.2 between the beam splitter 112 and object 101 will vary.
Assume here that the light modulator 114 is not provided in the interference optical system 110. In this case, an interference light is produced as will be described below. For instance, when there is no optical-path difference between the distances l.sub.1 and l.sub.2, the reflected light from the reference mirror 113 is in phase with that from the object 101, so that the interference light will be bright. When the object 101 moves so that the optical-path difference between the distances l.sub.1 and l.sub.2 is .lambda./2 (.lambda. is the wavelength of laser beam), the reflected light from the reference mirror 113 is in phase opposition to that from the object 101, thus the interference light will be dark. Therefore, if the light modulator 114 is not provided in the interference optical system 110, an interference light will take place in which bright and dark interference fringes develop each time the object 101 moves over a distance of .lambda./2.
On the other hand, in the conventional displacement measuring apparatus 100 having the light modulator 114 provided in the interference optical system 110, the light modulator 114 modulates the phase of the laser beam. Thus, an interference light produced in this displacement measuring apparatus 100 will cause bright and dark interference fringes corresponding to modulation frequencies even if there is no optical-path difference between the distances l.sub.1 and l.sub.2. The interference fringes of the interference light will change correspondingly to the optical-path difference between the distances l.sub.1 and l.sub.2 if the optical-path difference varies.
Therefore, in the conventional displacement measuring apparatus 100, as the object 101 moves, an interference light can be produced in which bright and dark interference fringes corresponding to modulation frequencies vary.
In the conventional displacement measuring apparatus 100, after such an interference light is detected by the photodetector 115, the interference fringes in the interference light are counted by the fringe detector circuit 105 to detect a moved distance, or displacement, of the object 101.
However, since the conventional displacement measuring apparatus 100 of the fringe counting type is adapted to count the interference fringes in the interference light, the resolution of displacement detection depends upon the wavelength of the laser beam. Therefore, the conventional apparatus can hardly detect a displacement of the object 101 with a high resolution.